Trees are saving more than 850 human lives each year and preventing 670,000 cases of acute respiratory symptoms in the U.S., according to the first broad-scale estimate of trees' air pollution removal by U.S. Forest Service researchers. Looking at four common air pollutants — nitrogen dioxide, ozone, sulfur dioxide, and particulate matter with a diameter less than 2.5 microns — researchers valued the human health benefits of the reduced air pollution at nearly $7 billion annually in a study published in the journal Environmental Pollution. The benefits of trees vary with tree cover across the nation, the researchers note. Tree cover in the United States is estimated at 34.2 percent overall, but varies from 2.6 percent in North Dakota to 88.9 percent in New Hampshire. While the pollution-removal capabilities of trees equaled an average air quality improvement of less than 1 percent, the impacts of that improvement are substantial, the study found.